High-density lipoproteins classification

Plasma lipoproteins are generally classified by their density and separation achieved with ultracentrifugation. According to this density-based classification system, the major lipoprotein classes are chylomicrons (CH), very low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). 

These lipids are insoluble in water and are classified on the basis of their ultracentrifugal properties into chylomicrons, very low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and high-density lipoprotein (HDL) in order of ascending density. Table 2.4 gives the classification and roles of lipoproteins. 

Other classes sometimes included are intermediate density lipoprotein ODL) and very-high-density lipoproteins (VHDL) (Schjeide SAjeide, 1981 Hermier et al, 1984 1985). There is some variation in the protocol for fractionation used by different research groups and ako in the density range used for classification. All three main fractions contain characteristic proteins and have been assigned particular functions. 

The lipoproteins present in plasma may be operationally defined according to their density, as low or very low density lipoproteins and high density lipoproteins, but other, more functional definitions may be more appropriate in connection with in vivo metabolism. Examples are the hpoprotein classes chylomicrons and chylomicron remnants. Alaupovic has suggested a classification based on the apolipoprotein composition [1]. As apolipoproteins often determine the metabolic fate of lipoprotein particles this is a logical approach. Lipoprotein particles with specific apolipoprotein compositions exist in all density fractions of human plasma, as well as plasma from a variety of animal species. 

All lipoprotein classes contain proteins, free and esterified cholesterol, TG and phospholipids however, the relative proportion of any component varies so that protein and phospholipid percentages are higher in a-lipoproteins (high-density lipoproteins) and lower in chylomicrons. The reverse is true for TG, while cholesterol circulates mainly as -lipoprotein (low-density lipoprotein). Since lipids circulate as lipoproteins, hyperlipaemias can be more properly defined as hyper-lipoproteinaemias. A classification of human hyperlipoproteinaemias based on chemical determination of plasma lipid classes as well as on paper electrophoretic separation of plasma lipoproteins has been proposed... 

LDL, low-density lipoproteins CHD, coronary heart disease. Classification <130 mg/dL is the desirable LI3L cholesterol level 130-159 mg/dl is borderline-high-risk >160 mg/dl is high-risk. 

The classification into chylomicrons (CM), very low density (VLDL), low density (LDL), and high density (HDL) lipoproteins is based on their relative contents of protein and lipid that determine the densities of these lipoprotein classes. CM have only 1-2% protein while HDL have about 50% protein by weight. The diameters of lipoproteins are inversely correlated with their densities and range from about 6000 A for CM to 70 A for the smallest HDL (Fig. 1). 

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